The aim of this proposal is to define the role of Bicaudal-C homolog 1 (Bicc1) in bone. Using an innovative systems genetics approach in the mouse we predicted that Bicc1 was the basis of a BMD quantitative trait locus (QTL). Bicc1 is an RNA-binding protein that has been implicated in the regulation of primary cilia. In this proposal, we demonstrate that jcpk mice, which are heterozygous for a Bicc1 null allele, are osteopenic and that genetic variants in human BICC1 gene are associated with BMD. Moreover, Bicc1 is highly expressed in differentiating osteoblasts and Bicc1 knockdown in primary calvarial osteoblasts impairs differentiation. We also show that Bicc1 regulates Pkd2, which is thought to be a critical component of the primary cilia on osteoblasts. Based on these data we hypothesize that Bicc1 influences BMD through an osteoblast and Pkd2 dependent mechanism. In Specific Aim 1 a series of in vitro and in vivo interaction experiments will be used to determine if the actions of Bicc1 on osteoblast differentiation and BMD are via the regulation of Pkd2 levels. In Specific Aim 2 we will determine if Bicc1 regulates BMD in an osteoblast-specific manner. This will be accomplished by ablating Bicc1 in osteoblasts using the cre-loxP technology and characterizing bone mass using microCT and histomorphometry. Transcriptional network analysis will also be used to further characterize Bicc1 function. The proposed studies will significantly advance our understanding of a novel BMD gene.